It is believed that there are global warming effects that are being caused by the introduction of increased carbon dioxide into the atmosphere. One major source of carbon dioxide emission is the flue gas that is exhausted as a result of a power generation plant's combustion process. Therefore, there have been several efforts by governments and utility companies worldwide, to reduce these emissions.
There are two principal types of power plants that are based on combustion processes: coal combustion and natural gas combustion. Both of these processes produce carbon dioxide as a byproduct when generating power. Efforts have been made to increase the efficiency of the burner, and therefore the basic combustion process itself. The intent of these efforts has been to reduce carbon monoxide (the result of imperfect combustion), oxides of nitrogen, and other pollutants. However, since the production of carbon dioxide and water are the basic products of the chemical reaction of combustion, the most efficient technique to minimize the carbon dioxide emission is to capture as much of the carbon dioxide as possible as it is being created by the power plants. In order to truly maximize the efficiency of this technique, existing coal combustion plants, which represent a large portion of the power generation plants worldwide, must also be targeted. The oxy-combustion technique is very interesting, and has significant advantages, since it can be adapted to existing facilities.
Traditional power plants use air as the source of oxidant to combust the fuel (typically coal). Steam is generated by indirect heat exchange with the hot combustion products. The steam is then expanded in turbines to remove useful energy and thereby produce power. The combustion process produces carbon dioxide as a by-product, which is mixed with the residual nitrogen of the combustion air. Due to the high content of nitrogen in the inlet air (78 mol %), the carbon dioxide is diluted in the flue gas. To insure full combustion, the power plants must also run with an excess air ratio that further dilutes the carbon dioxide in the flue gas. The concentration of carbon dioxide in the flue gas of an air combustion plant is typically about 20 mol %.
This dilution of the carbon dioxide increases the size and the power consumption of any carbon dioxide recovery unit. Because of this dilution, it becomes very costly and difficult to recover the carbon dioxide. Therefore, it is desirable to produce flue gas with at least about 90% to 95 mol % carbon dioxide, in order to minimize the abatement cost. The current technology for carbon dioxide recovery from flue gas utilizes amine contact tower to scrub out the carbon dioxide. However, the high amount of heat that is needed to regenerate the amine and extract the carbon dioxide reduces the amine processes cost effectiveness.
In order to avoid the dilution of carbon dioxide in the nitrogen, the power generation industry is switching to an oxy-combustion process. Instead of utilizing air as an oxidant, high purity oxygen (typically about 95% purity or better) is used in the combustion process. The combustion heat is dissipated in the recycled flue gas concentrated in the carbon dioxide. This technique makes it possible to achieve a flue gas containing between about 75 mol % and 95 mol % carbon dioxide. This is a significant improvement over the previous concentration of about 20 mol %, which is obtained with air combustion. The purity of carbon dioxide in oxy-combustion's flue gas ultimately depends on the amount of air leakage into the system and the purity of oxygen being utilized. The necessary high purity oxygen is supplied by an air separation unit.
Since pure oxygen, hence power input and capital cost, is required in the oxy-combustion process to facilitate the capture of carbon dioxide, the whole process, including the oxygen plant, the power plant itself, and related integrated concept must be very efficient to minimize the power consumption. Otherwise, the economics of the carbon dioxide recovery will become unattractive to the operator of the power generation plant. In summary, the carbon dioxide capture with oxy-combustion is appealing in terms of pollution abatement, however in order to achieve it, the capital expenditure and the power input must be minimized to avoid a prohibitive increase in power cost.
Over the years, there have been numerous efforts to reduce the cost of the air separation plant and in particular the production of oxygen. Since free atmospheric air is used as feed for the plant, the cost of oxygen is directly related to the power consumption and equipment cost of compressors, cold box, distillation, purification. With large quantities of oxygen being used for power generation (oxy-combustion and IGCC) oxygen plant size is increasing rapidly to 7-10 thousand tonnes per day requiring multiple trains of oxygen. In petrochemical applications (partial oxidation, Gas-to-Liquid, Coal-to-Liquid etc.), the need for oxygen is very large and reaching 20-30 thousand tonnes per day.
It is typical for oxygen plant used in oxy-combustion for pulverized coal power plant that low purity oxygen at about 95 mol % is the main product and there is no need for nitrogen gas. Without nitrogen production, it is not possible with traditional technique to co-produce pressurized nitrogen to improve the efficiency of the system. The power consumption of such oxygen plant is directly related to the air pressure feeding the cryogenic distillation column system. The lower the pressure, the more efficient the air separation plant. For this type of application, the air pressure is usually about 3-4 bar abs.
Waste nitrogen from such oxygen plant must be generated at low pressure for atmospheric venting in order not to waste pressure energy. The piping and exchangers of waste nitrogen circuit must therefore be oversized to have low pressure drop otherwise the air pressure must be raised, hence resulting in higher power consumption.
By operating the air separation process at low feed air pressure at about 3-4 bar abs., the power consumption can be minimized when compared with traditional oxygen plants operated at 5-6 bar abs. However, at low feed air pressure, the front end adsorption equipment for moisture and carbon dioxide removal becomes problematic since the lower the pressure, the more moisture is carried in the feed air. Adsorption is exothermic and with more moisture being adsorbed, higher temperature rise in adsorbent bed cannot be avoided, which is not favorable for adsorption. Adsorption capacity is also further reduced at low pressure due to low partial pressure of CO2, requiring lower temperature air cooling equipment to minimize the adsorbent quantity.
It can be seen from the above description that the air separation equipment cost increases rapidly when the air pressure is lowered to reduce power consumption. Low pressure drop results in expensive voluminous piping, exchanger, and distillation columns. Adsorption equipment becomes very large and very costly.
As mentioned above, the plant size requirement is also increasing and the larger equipment, in particular the distillation columns and adsorption bottles, exceeds the limit of transportability. Cryogenic equipment capacity must be reduced to cope with maximum transportable equipment size such that higher number of trains is needed. It is obvious higher equipment and installation costs will occur.
If the nitrogen product of the low pressure column can be utilized at elevated pressure (for example as in the case of integrated oxygen plant for Integrated Coal Gasification Combined Cycle (IGCC) application) then an alternative solution would be the elevated pressure process. In this process, the low pressure column's pressure is raised to 3-6 bar abs instead of 1.3-1.6 bar of the low pressure plant. Due to the elevated pressure of the low pressure column, the feed air pressure becomes much higher at about 10-16 bar abs. When compared with the low pressure solution, the elevated pressure air separation process offers several advantages:                more compact front end purification unit due to higher feed air pressure        smaller distillation columns, especially the low pressure column, due to higher operating pressure        smaller heat exchangers and piping due to higher operating pressure        more compact equipment means more capacity per shippable train        
The main penalty of the elevated pressure process is the much higher power consumption due to higher air pressure. If the pressure of nitrogen product from the plant can not be valorized, or if there is no need for pressurized nitrogen, then this approach cannot be justified economically under most circumstances due to the expense of the additional power consumption.
It is useful to note that the separation energy of oxygen for elevated pressure process is about 0.2-0.24 kWh/Nm3 or 20-30% better than the separation energy of low pressure plant (about 0.26-0.30 kWh/Nm3), taken into account the energy credit of nitrogen product. This type of elevated pressure air separation process is described in many publications and patents such as U.S. Pat. Nos. 4,224,045, 5,081,845, 5,421,166, 5,231,837, 6,116,052 etc.
Because of the advantages of elevated pressure cycle, there exist many techniques developed to efficiently recover the energy of pressurized nitrogen in order to utilize this cycle when pressurized nitrogen is not required.
U.S. Pat. No. 3,950,957 describes a process wherein the nitrogen from the elevated pressure oxygen plant is heated by recovering heat from the air compressor outlet. The nitrogen is further heated by the flue gas of an air and fuel combustion of a steam generator; it is then expanded in a turbine for power recovery. The exhaust of the turbine is sent back to the steam generator for further heat recovery. By exchanging low level heat at the outlet of the turbine with high level heat supplied to heat the nitrogen prior to expansion, thermal efficiency of the boiler can be preserved and additional power can be recovered from the turbine to drive the air compressor.
U.S. Pat. No. 4,224,045 describes the injection of the nitrogen into the gas turbine to recover its pressure energy.
U.S. Pat. No. 5,040,370 describes an arrangement wherein the oxygen of an air separation plant is used in an external process to produce a hot stream of fluid of temperature less than 600° C., which is used to heat nitrogen prior to expansion for power recovery.
U.S. Pat. No. 5,076,837 describes a similar approach as U.S. Pat. No. 5,040,370 for application with a chemical process.
U.S. Pat. No. 5,317,862 proposes moisturizing and heating nitrogen then expanding it to recover power for blast furnace application. Almost all of the techniques were developed using integration with a gas turbine to recover the pressure energy of the nitrogen produced by the elevated pressure process.
U.S. Pat. No. 5,388,395 proposes to expand nitrogen for power recovery. Power savings is further achieved by mixing the chilled nitrogen of the turbine exhaust with gas turbine's air inlet to lower the inlet temperature.
U.S. Pat. No. 5,635,541 proposes to use the elevated pressure plant for remote gas process such as Gas-to-Liquid (GTL) to minimize equipment cost, the nitrogen is simply expanded for power recovery.
U.S. Pat. No. 6,009,723 suggests expanding the heated nitrogen to drive some ASU's compressor.
U.S. Pat. No. 6,263,659B1 suggests heating nitrogen by heat exchange with the gas turbine exhaust then expanding in case there is no combined steam cycle.
U.S. Pat. No. 6,282,901 describes a process that introduces pressurized oxygen and fuel on the shell side of the heat recovery boiler, then work expand the resulting flue gas recover energy.
In coal combustion boiler the hot flue gas at about 2000° C. generated from the combustion of coal with oxygen is used to vaporize boiler feed water at high pressure to produce steam. Steam is then expanded to low pressure (vacuum) in multiple steam turbines to produce electricity. The boiler utilizes very hot flue gas to generate superheated steam as high as 570° C. Because of the large temperature difference, the ratio of flue gas flow to the steam flow is quite small. By vaporizing steam at multiple pressures, the low flow ratio of flue gas can be optimized for high temperature water vaporization. However, at the low temperature range, the low flow ratio of flue gas is not sufficient to preheating the boiler feed water and providing heat for de-aeration. In fact, in case of supercritical steam cycles, almost 70% of the total heat duty is provided for this medium and low grade heat up to the critical temperature of water at about 340° C. To complement for this shortage, steam extraction at various interstage levels are required. This steam extraction, even at low pressure, deprives the turbines of the expanding steam, hence reducing the power output of the power plant and its thermal efficiency. A typical arrangement for steam extraction and heating of boiler feed water is illustrated in FIG. 1.
Based on the above there is a need to provide an integration process between the oxygen plant and the oxy-combustion coal power plant such that an elevated pressure oxygen process can be used to reduce the capital cost of the oxygen plant. The thermal integration must also provide efficient power recovery of the pressurized nitrogen and improving the performance of the boiler.